The present invention is directed to integrated circuits. More particularly, the invention provides systems and methods for protecting a power conversion system. Merely by way of example, the invention has been applied to protecting a power conversion system with pulse-width modulation. But it would be recognized that the invention has a much broader range of applicability.
Pulse-width-modulation (PWM) technology is widely used in power conversion systems. Various protection mechanisms, such as over-voltage protection, over-temperature protection, current-limiting or over-current protection (OCP), and over-power protection (OPP), are often built in circuitry associated with a PWM controller to protect a power conversion system in which the PWM controller is used from potential damages. The protection mechanisms usually work when the circuitry associated with the PWM controller operates in normal conditions. But when the circuitry associated with the PWM controller is under certain conditions, the protection mechanisms often cannot function properly.
FIG. 1 is a simplified diagram showing a conventional power conversion system including a pulse-width-modulation (PWM) controller. The power conversion system 100 includes a PWM controller 102, a power switch 124, a current-sensing resistor 126, an isolated feedback component 128, a primary winding 130, a secondary winding 132, a capacitor 134, and a rectifying diode 136. The PWM controller 102 includes a PWM component 104, a logic-control component 106, a gate driver 108, a comparator 110, and a leading-edge-blanking (LEB) component 112. Further, the PWM controller 102 includes five terminals 114, 116, 118, 120, and 122. For example, the power switch 124 is a transistor.
In operation, a primary current 138 flows through the primary winding 130, the power switch 124, and the current-sensing resistor 126 (e.g., Rs). In response, a resistor signal 140 (e.g., VCS) is output to the terminal 118 (e.g., terminal CS). The LEB component 112 receives the signal 140, and outputs a current-sensing signal 142 to a non-inverting input terminal of the comparator 110. For example, the current-sensing signal 142 is the resistor signal 140 processed by the LEB component 112. In another example, the LEB component 112 can be removed, and the current-sensing signal 142 is the same as the resistor signal 140.
The comparator 110 receives a threshold signal 144 (e.g., Vth-OC) at an inverting input terminal, and generates a comparison signal 146 based on the threshold signal 144 and the current-sensing signal 142. The PWM component 104 receives a feedback signal 150 from the isolated feedback component 128 and the current-sensing signal 142, and in response generates a modulation signal 152. The logic-control component 106 receives the comparison signal 146 and the modulation signal 152, and in response outputs a signal 148 to the gate driver 108 for driving the power switch 124.
Over-current protection is usually needed to limit the primary current 138 in order to protect the power conversion system 100 from various damaging conditions, such as excessive power, thermal run-away, transformer saturation, and excessive current and voltage stress. In normal operation, the primary current 138 is lower than a predetermined current limit (e.g., ILimit) in magnitude. The predetermined current limit can be determined according to the following equation.
                              I          Limit                =                                                            V                in                                            L                p                                      ×                          t              on                                =                                    V                              th                ⁢                                  -                                ⁢                OC                                                    R              s                                                          (                  Equation          ⁢                                          ⁢          1                )            where ILimit represents the predetermined current limit, Vin represents an input voltage 154 on the primary winding 130, and Lp represents an inductance 156 of the primary winding 130. Additionally, ton represents a time period during which the power switch 124 is closed (e.g., on), Vth-OC represents the threshold signal 144, and Rs represents the resistance of the current-sensing resistor 126.
If the primary current 138 becomes greater than the predetermined current limit (e.g., ILimit) in magnitude, the current-sensing signal 142 is greater than the threshold signal 144 (e.g., Vth-OC) in magnitude. In response, the PWM controller 102 turns off the power switch 124, and shuts down the power conversion system 100. But under certain conditions, the power conversion system 100 often cannot be effectively protected from being damaged or blown out.
Hence it is highly desirable to improve techniques for protecting a power conversion system.